It depends on what they want from the site. Normally I'd say choose something they are interested in, and that won't be too much work to update regularly. That way for a first site, they can try out coding skills and similar without committing too much money or time to it.
I would say pick a niche that you are familiar with, enjoy and are good at yourself. Getting your first website up is hard enough from a technical standpoint, may as well make it easy with this part of it.
I would suggest you to pick a content that you re interested in and also have good knowledge in that subject, that makes it easy for you to build a great website because its easy to write about something that you already know.
Any niche will make you money as long as you know what you're doing. Some people spend way too much time trying to decide on the 'best' niche to make money and there is no such thing. Sometimes even the oddest things do, remember the Pet Rock phenomenon? Something totally useless but somebody made millions off it. Just pick something that you are going to still be interested in years from now.
If you are interested to challenge yourself a bit, think further beyond things you like and see the trend. Talking about technology has a better prospect than building a website related to a specific TV show which will end one day for sure. I still think that you should love doing what you do assuming you are ready to do this for a long time.